1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery.
2. Description of the Related Art
A nonaqueous electrolyte battery in which lithium metal, a lithium alloy, a lithium compound or a carbonaceous material is used for forming the negative electrode is expected to provide a battery of a high energy density, and vigorous research is being conducted in an attempt to develop the particular nonaqueous electrolyte battery. A lithium ion battery comprising a positive electrode containing LiCoO2 or LiMn2O4 as the active material and a negative electrode containing a carbonaceous material that intercalates-releases lithium has been widely put to the practical use. Also, use of a metal oxide or an alloy in place of the carbonaceous material in the negative electrode is being studied. Particularly, in the case of mounting the battery in a vehicle such as an automobile, the negative electrode is required to be formed of a material excellent in chemical and electrochemical stability, in mechanical strength and in corrosion resistance in view of cycle performance in a high-temperature environment and reliability of high rate discharging over a long time. On the other hand, a nonvolatile and incombustible electrolysis solution, which is being developed as the nonaqueous electrolyte in view of improvement in safety, has not yet been put to practical use because the high-rate characteristics and the long life performance are reduced.
Various studies are being made in an attempt to improve the negative electrode characteristics. For example, Japanese Patent Disclosure (Kokai) No. 2002-42889 teaches that a negative electrode prepared by allowing a specified metal or alloy to be supported by a current collector formed of aluminum or an aluminum alloy is used in a nonaqueous electrolyte secondary battery.
In the secondary battery of the type noted above, the current collector included in the negative electrode does not have a sufficiently high mechanical strength. Therefore, if the negative electrode is made thinner to increase the density of the negative electrode, it is possible for the battery to be markedly restricted in battery capacity, the high-rate performance, cycle life and reliability. Also, if the particle diameter of the active material of the negative electrode is increased in place of decreasing the thickness of the negative electrode, the interface resistance of the negative electrode is increased so as to make it more difficult to bring about a high performance.
On the other hand, Japanese Patent Disclosure No. 2001-143702 teaches that secondary particles having an average particle diameter of 5 to 100 μm, which are formed by the agglomeration of primary particles of a lithium titanate compound represented by LiaTi3-aO4 (0<a<3), the primary particles noted above having an average primary particle diameter smaller than 1 μm, are used as the active material of the negative electrode. It is taught in this connection that the agglomeration of the secondary particles is suppressed, and that the yield of manufacture of the negative electrode for a large battery having a large area can be increased.
The battery disclosed in Japanese Patent Disclosure No. 2001-143702 referred to above certainly makes it possible to suppress the agglomeration of the secondary particles. However, since the primary particles are agglomerated, the irregularity on the surface of the negative electrode becomes rough, thereby decreasing the surface area of the negative electrode. In addition, the battery in this prior art is defective in that the affinity of the negative electrode with the nonaqueous electrolyte is lowered so as to shorten the charge-discharge cycle life of the battery.